One parent approved, the other refused, on the same papers
Each application is decided on its own, by whichever officer picks it up, and a couple does not travel as a single unit in law. The refused parent is usually the one with less in their own name: no pension of their own, no property in their own name, no independent travel history. On paper they read as a person with nothing to return to, because everything they would return to belongs to their spouse.
Build each application as though the other did not exist. Joint assets need documents that name both people, and the parent with less in their own name needs their ties evidenced directly rather than inferred from the marriage.